Hematopoietic stem cells for use in transplantation are collected by a procedure called leukopheresis. In general, the donors of stem cells are normal subjects whose stem cells will be used to repopulate the bone marrow of patients with disorders such as leukemia, lymphoma, or aplastic anemia. At times, however, stem cells are collected from patients with diseases who then undergo high-dose chemotherapy, followed by reinfusion of their own stem cells to "rescue" their bone marrow from the toxic effects of treatment. This project is aimed at providing hematopoietic stem cell samples to laboratory investigators interested in defining the genetic regulation of stem cell differentiation and the engraftment potential of various subsets of stem cells. Stem cells from both normal donors and patients are collected by leukopheresis for clinical purposes, but an aliquot of each stem cell preparation is then made available to laboratory investigators for study.